In offshore oil and gas drilling, production, and completion operations a platform at the surface of the ocean is connected to the well head on the sea floor by risers. A riser is a tubular member through which drilling tools, tubing, and other components used in oil and gas exploration pass. The current practice is to make the risers from steel. More recently, it has been proposed that the risers be made from composite materials. Risers made from a composite material offer the advantage of being lighter in weight than steel risers. Thus, composite risers have the advantage of requiring a smaller surface platform to support the same length of composite riser than would be required with a steel riser.
Offshore oil and gas exploration is progressively moving to deeper and deeper water. Thus, the weight savings advantage of the composite riser become more significant as the water depth in which wells are drilled becomes greater. Some well heads are on the sea floor more than 5,000 feet below the surface of the ocean.
A concern with any deep water oil and gas exploration is maintaining the integrity of the riser system. Breaches in the riser system can result in the escape of drilling muds, oil and/or gas into the sea.
The use of composite risers in actual field applications is relatively new. Thus, there is little long-term experience concerning the reliability of composite risers. Clearly, failure or breach of a riser is to be avoided. The present invention provides an apparatus and method for monitoring the integrity of composite risers by monitoring changes in the riser stiffness. Monitoring of the stiffness of the risers can allow identification of weakened risers and allow their replacement prior to failure. A change in the stiffness is monitored using strain sensors or vibration sensors.
Stiffness is defined as a measure of the amount of deformation per unit load. When a riser joint is new, it will have certain stiffness value and therefore when the joint is subjected to a certain load, the joint will deform to a certain level, which can be measured using displacement gauges or strain sensors. The strain is defined as the displacement per unit length of the section over which the displacement is measured. The virgin stiffness of a riser joint can be predicted using numerical solutions and the amount of strain when the riser joint is subjected to a specific load can also be predicted using numerical solutions such as finite element analysis. When the riser is damaged, the stiffness will be reduced and the amount of deformation for the same load will be increased.
Stiffness of the composite riser is an important design parameter because high stiffness results in high loads when the riser stretches as the platform moves and low stiffness is not desirable because it can result in clashing between different risers. The axial stiffness of the riser is related to the elastic modulus of the riser, the cross sectional area and the length of the riser string. The length of the riser string is defined by the water depth and the cross sectional area is mainly established to ensure that the riser can withstand the design loads such as pressure, tension and bending loads. The elastic modulus is affected by the fibers used to manufacture the composite riser and the layout of the different laminates. While the currently used material, steel, has a fixed elastic modulus of 30 million lb/square inch (206.85 million kPa), composite risers can have different values. The present invention can be used with composite risers, the elastic axial modulus of which is between 5 to 15 million lb/square inch (34.475 and 103.425 million kPa), and preferably a value between 10 and 14 million lb/square inch (68.95 and 96.53 million kPa). Damage to the composite riser will manifest itself by a reduction of the riser's stiffness, indicating that the elastic modulus of the riser has been reduced.
It is also noted that the composite riser joint will fail when the strain in the riser reaches a specific value. This value is in the order of 0.5% for the carbon fiber composite risers being considered for offshore applications. An object of the present invention is to monitor riser strain either (1) on a continuous basis to assess the extent of damage and also the variation of loading, or (2) by monitoring for the maximum strain experienced in the riser until it reaches a specific value which is lower than the strain at which failure is expected. This will ensure sufficient time to remove the damaged joint prior to its failure. In another aspect, the present invention provides for using the natural vibration frequency of the riser to monitor the integrity of the riser. As the stiffness of the riser changes, its natural frequency, which is a function of the riser's stiffness and mass, will change and thus the riser's vibration signature will change. Although this is a well known technique, individual testing and the generation of custom strain curves is required to characterize a specific riser because configuration, cross-section, wall thickness, material selection, etc. will affect vibration response characteristics. Monitoring the changes in a riser's vibration signature, which is commonly done using accelerometers, can provide an indication of the level of damage to that riser. Because of the complexity of the composite structure, theoretical predictions of the relationship between level of damage and changes in strains or vibration signature are difficult. Therefore, calibration curves need to be developed as part of the riser qualification program. Developing these curves involves testing some composite joints to induce damage. In one embodiment of the invention, fiber optics are used as the strain sensors and a test method is provided demonstrating the qualification of the riser when strain monitoring is used.